In many applications, an operator controls a remote image sensor via a communication link. Typical operator controlled applications are traffic control, border control, search and rescue operations, land surveys, police surveillance, etc. Operators may additionally request measurements of the remote tracked object, such as motion parameter measurements and the like.
Reference is now made to FIG. 1, which illustrates a prior art surveillance/tracking system 10, comprising a remote image-sensing unit 20 and a control center 22, which are connected via a communication link 25.
Sensing unit 20 comprises a sensor communication unit 28, a remote tracker 30, an image sensor 32, and a pointer control unit 34. The methods of operation for these elements are well known in the art. Sensing unit 20 locates and tracks a sighted object, and transfers sensor data 24, such as image pictures, track location, pointing data and the like, to control center 22. Sensor data 24 travels in the direction represented by arrow A.
Control center 22 comprises a control communication unit 36, a display 38 and a control stick 40. The methods of operation for these elements are also well known in the art. Control center 22 provides control data 26, such as pointing instructions, centering instructions, track commands, track corrections and the like, to sensing unit 20. Control data 26 travels in the direction represented by arrow B.
Communication link 25 is connectable to sensing unit 20 and control center 22 via sensor communication unit 28 and control communication unit 36, respectively. Furthermore, communication link 25 transfers sensor data 24 and control data 26, via sensor communication unit 28 and control communication unit 36, respectively.
Generally, image sensor 32 surveys an object, and relays image pictures (sensor data 24) to display 38, which displays the pictures for viewing by an operator 42.
If operator 42 decides that it is desirable to track the object, he sends via stick 40 manual coarse pointing instructions (control data 26), such as “move up”, “move right”, “zoom” and the like, to pointer control unit 34. Pointer control unit 34 acts upon these instructions, and directs image sensor 32 in the instructed direction.
Operator 42 then sends via stick 40 centering instructions to pointer control unit 34. Pointer control unit 34 directs image sensor 32 in the instructed direction, thus centering the object in the center of the field of view (FOV) of display 38. Once the object as sensed by imaging sensor 32 is centered in the FOV, operator 42 electronically sends via stick 40 locking instructions to remote tracker 30. Remote tracker 30 receives the instructions and attempts to lock onto the object in the center of the FOV of display 38.
Once the object has been locked, remote tracker 30 takes over command and generation of the tracking operation. Pointer control unit 34 ceases to receive commands via stick 40 and instead commences to receive instructions from tracker 30. Upon receipt of the instructions, pointer control unit 34 relays them to the image sensor 32. Image sensor 32 tracks the moving object and keeps the object in the center of FOV of display 38, even while the object moves relative to sensing unit 20.
It is noted that there are various known in the art methods and apparatus for identifying and tracking an object. Among these methods are center of gravity identification, edge detection, correlation, and other known in the art methods.
In many applications, there is a considerable time delay between the time when sensing unit 20 acquires an image picture of an object, to when the image is displayed on display 38, and finally, to the receipt of the responding instructions by sensing unit 20. Generally, the main factors contributing to the delay are signal processing, image compression/decompression, and/or link bandwidth limitations. Consequently, when taking into account the delayed reaction time of the operator, the accumulated delayed time can from hundreds of a second to several seconds.
Due to the time delay, the location of the image as displayed on display 38 is generally not the current location of the object. The location displayed on the screen is the location of the object before the transfer of the sensor data 24, (e.g. A seconds ago). Additionally, by the time the pointer control unit 34 receives the instructions (control data 26) additional time has lapsed, (e.g. an additional B seconds). Subsequently, by the time image sensor 32 is instructed to locate the object, the object may not longer be in the same location it was when the image picture was taken over A +B seconds ago.
Clearly, this time delay complicates the efforts to lock remote tracker 30 onto the object. Operator 42 has to accurately calculate and estimate the expected location of the object at the time in the future when the tracking instructions are to arrive at sensing unit 20. Only then is pointing control 34 pointed to the calculated estimated location, and remote tracker 30 instructed to lock and initiate tracking.
If the estimated location calculation is not accurate enough, remote tracker 30 will lock onto some other background object and the entire estimate, calculate and lock process has to be repeated. As such, the effect is a continuous feedback control loop with delay, a situation which is liable to suffer from overshoots and instability.
The locking process is complicated even more by the human input in the tracking loop. Human reactions and directions are less precise than, as an example, computer or processor generated instructions. Humans do not function well in feedback loops with time delay, such an example being the typical daily experience of adjusting the temperature of hot water from a faucet with a slow reaction time.
One solution to the time delay problem is to minimize the delay and/or optimize the control circuitry bandwidth so that overshoots and oscillations are minimized. Another solution is to use a prediction cursor which measures the dynamics of the sensor, and calculates its predicted position at the time of receipt of the lock instruction. Additionally, operators can be trained to optimally estimate the location of the object at time of execution of the lock instruction.